By bombarding a thick iron target with a high-power laser, a broad-band source of X-rays has been produced. The conversion efficiency of 1.06-micron laser light into X-rays is at least 10%, probably 15-20% and possibly 30-40%.
Above 1 kV the output was mainly spectral lines, and the major portion below 1 kV was probably lines, too. At energies above 1 kV, the conversion efficiency is about 5%.
The result can be a big step forward toward a brute-force X-ray laser. One possibility is to use a broad-band X-ray source of very high intensity as a laser pump.
A Hadron CGE-640 neodymium-doped glass laser is pulsed at 100 joules in 1 nanosec. The beam passes through an f/1 or f/3 lens, strikes a target at 45.degree. with respect to the incident beam, and a plasma is produced in the target at the laser focus. The target is a 1-cm-thick slab, essentially infinitely thick to the laser. Targets of chromium, calcium, nickel, aluminum, lead, tungsten, and gold provide X-ray conversion with varying efficiencies. With chromium essentially the same efficiency is obtained as with iron.
The distribution of internal states in the atom, rather than the gross properties of the target, are what determines the output. That is, the spectrum and conversion efficiency depend on the atomic number Z of the target, and one can expect large conversion efficiencies from any material with Z above 10, provided one properly adjusts the precursor pulse or foot, a low-power pulse that precedes the main pulse. The purpose of the foot is to produce plasma so that the main pulse will strike a plasma rather than a solid.
For iron, the material used for most of the studies, with no foot one gets very soft X-rays. With a long foot one gets a few harder X-rays. With full parametric variation of foot size one gets an optimal value at 8-10 nanosec pulse duration.
Using a computer analysis to solve the rate equations resulting from a microscopic description of all the principal collisional and radiative processes in the experiment, good agreement is found between the calculation and experiment.
Experiments were made also with pellets, but most of the emphasis has been on slabs because they appear to be more useful in gaining basic data, although they may not be the best geometry. Similarly, emphasis has been on iron, although it may not be the best material.
Laser bombardment of high-Z materials appears promising for an X-ray laser, and also has other used. In fact emphasis has primarily been on other uses.
Because of the good conversion efficiency and considerable line output, it appears that by making a different selection of targets, most of the energy could be made to come out in a few lines. When one pulses a material rapidly compared to the lifetime of the states involved, one would expect to produce metastable states. If superradiant levels are present in the laser focus, the device would be close to a laser. The only problem left would be to make a cavity.
The interaction of a laser beam with high-Z materials is also useful for controlled fusion. The technique may play a key role in lowering the threshold laser energies for achieving fusion. Although most of the emphasis in this effort has been with light materials, high-Z materials could be used in conjunction with deuterium and tritium, perhaps in different layers.
An intense point source of X-rays with a high conversion efficiency of laser light into X-rays has been developed. The X-rays emanate from a spot of approximately 100 microns diameter, and a large fraction of the X-rays (sometimes 50% or more) are emitted in the form of spectral lines.
An important implication of these properties is the fact that many of the applications that are generally cited in justification of an X-ray laser can be accomplished with a point source (i.e., "spatially coherent" source) of X-ray lines, or even with a point source of broad band X-rays. For example, a significant fraction of the X-rays can be focused to another point or into a parallel beam by means of critical angle reflectors, Fresnel zone plates, or Bragg diffractors.
It is calculated that the X-ray emission in a single spectral line can exceed 1% of the indicent laser light. This can also be true when a substantial amount of amplification through stimulated emission occurs. In the latter case the device constitutes an actual X-ray laser, which can provide a narrow beam if the plasma shape is a suitably long cylinder or an X-ray cavity is constructed. Indeed, in addition to the fact that it can serve many of the functions of an X-ray laser, the point source device is only one step away from a true X-ray laser. Large outputs of X-ray lines are being produced in a highly nonequilibrium plasma, and theory says that a substantial amount of stimulated emission can occur with some adjustments of plasma temperature of target configuration.
Statements are often made that picosecond or terrawatt pumping pulses are required for an X-ray laser. This appears to be untrue. Calculations indicate that a nanosecond laser pulse with 100 to 1000 joules of energy should be more than adequate.
Another route to an X-ray laser is to use the X-rays generated by the present technique to knock out inner shell electrons from atoms in a separate laser medium and thereby create a population inversion.